The present invention relates to a new adhesive and a process for making wood laminates employing the adhesive.
A wide variety of adhesives and processes are known for preparing wood laminates. Among the most widely used adhesives are various phenol-aldehyde condensation products. Typically, these adhesives are applied to wood plies and the plies are then subjected to a pressing operation to consolidate them into a unitary panel and cure the adhesive. In many situations it is desirable to perform the pressing in two or more stages to maximize the output from a fixed number of heated platen presses. Typically, a first pressing stage will be performed at ambient temperature under a pressure and for a time just sufficient to consolidate the separate plies into a unitary panel. A panel consolidated in this fashion can be stored and handled without shifting or separation of the plies. The consolidated panels are then typically cured under heat and pressure to form the final laminate.
The use of phenol-aldehyde adhesives is quite common in the production of wood laminates. In U.S. Pat. No. 3,637,547 to Orth, a colloidal, acicular magnesium aluminum silicate clay, such as attapulgite and sepiolite, is added to an otherwise conventional phenol-aldehyde adhesive. These particular clays are added in amounts of from 5 to 50% based on the weight of the phenol-aldehyde solids. In Example I of the above patent, panels prepared with an adhesive including 45% bentonite clay had a dry wood shear failure of 50% and a wet wood shear failure of 35%, as compared to values of 90-100% and 90-95% for identical samples prepared with attapulgus instead of bentonite.
It has generally been preferred in the past to prepare wood laminates from essentially dry wood plies because moist veneers invariably produce panel blows on opening the hot press after panel curing. Moreover, adhesives of the phenol-aldehyde type cure by condensation and are therefore inhibited to a certain extent by the presence of water. Recently, however, more attention has been given to the production of laminates using wood plies, such as southern pine veneers, having moisture contents of from 0 to 14% or higher based on the dry weight of the wood. The use of high moisture content plies is advantageous because conservation of energy due to reduced drying time and therefore more economical. Unfortunately, conventional adhesives do not perform satisfactorily in this environment, and usually prolonged pressing times are incurred and an undesirable number of laminates must be rejected due to panel blows.